Radiocarbon Dating

  1. Radiocarbon (14C) dating is often used to obtain an absolute date for recent organic materials.
  2. Carbon-12 (12C) is a stable carbon isotope.
  3. Cosmic rays convert 14N to carbon-14.   14C is radioactive.
  4. The carbon in atmospheric carbon dioxide (CO2) is mostly 12C, but a very small amount is 14C.
  5. Living plants take up CO2. Plants are eaten by animals. Living organisms have a small amount of 14C in them.
  6. Although the 14C continually disappears (decays) in the organism, it is balanced by the uptake of more 14C.
  7. When the organism dies, the uptake of 14C ceases.
  8. The radioactive decay continues until all the 14C is converted back to N-14.
  9. We can estimate the age of the organism's death by the 14C / 12C ratio.
radiocarbon decay graph

Use the graph to help answer these questions:

  1. What is the definition of half-life?
  2. How many years does it take for 50% of the 14C to decay? ______
  3. How many years until only 10% of the 14C is left?_______
  4. If you have a plant specimen with 25% of its 14C left, how many half-lives have passed? ______
  5. How old in years is that plant specimen in the question above? ______
  6. If a bone artifact is 35,000 years old, what percentage of the 14C would be left? ______
  7. Explain why is the practical limit for dating with 14C about 40,000 years.